Yes, the vacuum was always there. I'll get pics up tomorrow. If it failed years ago, then wouldn't the old tank have the same problem? When I took it to the shop (I've had this petcock installed for months now), the mechanic mentioned something about the new tank missing a diaphragm that the old one did in fact have. He offered two fixes; the more expensive fix being installing a new diaphragm for the new tank or the cheaper, manual petcock which I went with.
There is no diaphragm in the tank. The vacuum fuel shut off has a diaphragm though. Maybe that is what your mechanic was referring to. The needle and seat should stop fuel from entering the carbs. That is it's job. A few reasons for the needle and seat to fail are dirt holding it open, wear and vibration. The fuel shut off is there in case the needle and seat fail or a tip over. The manual petcock is more reliable.
I often don't close the petcock though I know I should. No problems yet
